The present invention relates to a method for manufacturing an optical attenuators, and more particularly for manufacturing an optical attenuator by overlapping and welding two optical fibers and then making a mode filter.
An optical attenuator is an important component often used in optical fiber transmission systems. In the present state of the art, some methods for manufacturing optical attenuators have been disclosed. For example, a method is disclosed in U.S. Pat. No. 4,717,234, for attenuating light by inserting an absorption film in the path of light, to absorb a portion of the light by an absorption coating on the film. U.S. Pat. No. 5,095,519 discloses another method for attenuating light by inserting a piece of pure glass fiber between two optical fibers by welding, to scatter a part of light. Furthermore, a method for producing an optical attenuator is disclosed in U.S. Pat. No. 4,884,859 according to the scattering loss principle. In this method, an optical fiber is heated, then twisted or stretched before it is cooled down, to form a series of cracks in the heated region. Light will scatter in the region and therefore be attenuated. However, the above mentioned methods have a common shortcoming in that the attenuation of light is difficult to control.
A method for controlling the attenuation of light is disclosed in U.S. Pat. No. 4,557,556. According to the coupling principle of light, two optical fibers are offset for a short distance, and then welded. Light will be attenuated because the fibers are not completely coupled. The attenuation can be adjusted by adjusting the distance between two fibers core before they cool down. Since the fibers are axially coupled, this method requires ultra smooth end surfaces for the fibers which is very difficult to produce. In addition since the fibers must be axially aligned with high precision such precise alignment is difficult to achieve and results in an excessive number of defective products.
U.S. Pat. No. 5,285,516 discloses a method to produce an optical attenuator by overlapping and welding two fibers, which is suitable for mass production. However, its mode characteristic is unstable, leading to an unstable degree of attenuation, therefore it is not quite practical.